1. Field
Example embodiments relate to magnetic memory devices, methods of driving the same and data writing and reading methods for the same, and more particularly, to magnetic memory devices that may minimize or reduce magnetoresistance (MR) reduction and/or may reduce critical current density (Jc), methods of driving the same and data writing and reading methods for the same.
2. Description of the Related Art
As the information industry develops, the processing of relatively large amounts of information is required. Thus, the demand for data storing media capable of storing relatively large amounts of information is continuously increasing. As the demand for the data storing media increases, studies about relatively small-sized information storing media having relatively high data storing speed have been conducted. As a result of these studies, various kinds of information storing apparatuses have been developed.
Information storing apparatuses are either volatile information storing apparatuses or non-volatile information storing apparatuses. In the case of the volatile information storing apparatuses, stored information is erased when power is turned off. Volatile information storing apparatuses have higher writing and reading speeds than non-volatile information storing apparatuses. In the case of the non-volatile information storing apparatuses, stored information is not erased even though power is turned off.
A dynamic random access memory (DRAM) IS an example of a conventional volatile information storing apparatus. A hard disk drive (HDD) and a random access memory (RAM) are examples of conventional non-volatile information storing apparatuses. A magnetic random access memory (MRAM), which is a type of volatile information storing apparatuses, uses a magneto-resistance effect based on a spin dependent electron transport phenomenon.
Conventional magnetic memory devices switch a magnetization direction of a free layer of a memory cell using a magnetic field generated by a current flowing through a bit line and a word line of the conventional magnetic memory devices. However, this method has the following drawbacks.
When the size of a unit cell is reduced to realize a relatively high-density memory device, the coercivity of the free layer increases. As a result, a switching field increases. Thus, the magnitude of an applied current may be increased. Moreover, because a relatively large number of memory cells are included in a memory array structure, free layers of unwanted cells may switch. Thus, conventional magnetic memory devices that switch using the magnetic field have relatively low selectivity and may hardly realize high-density memory devices.
Magnetic memory devices that use a spin transfer torque (STT) phenomenon may address the above-discussed drawbacks of high density, selectivity, and high writing current, and thus, many studies have been conducted on magnetic memory devices using the STT phenomenon. In this conventional method, a free layer of the magnetic memory device is switched to a desired direction using a spin transfer of electrons by allowing a current in which spins are polarized in a direction to flow in the magnetic memory device. This conventional method is advantageous for realizing a relatively high density because the required current is relatively small as the cell size decreases. However, the critical current density required for switching the magnetic memory devices that use the STT phenomenon is too large for the magnetic memory devices to be commercialized, and thus, studies have been conducted to reduce the critical current density of the magnetic memory devices that use the STT phenomenon.
Various methods of reducing the critical current density have been proposed as follows.
In one example, a critical current required for switching may be decreased by increasing a polarization factor of an input current. However, the polarization factor is a basic property of a material, and thus, polarization factor is rarely increased. Alternatively, a multiple layer structure may be used.
FIG. 1 is a cross-sectional view of a structure of a conventional magnetic memory device.
Referring to FIG. 1, the conventional magnetic memory device includes a first anti-ferro-magnetic layer 101, a first pinned layer 102, a first non-magnetic layer 103, and a free layer 104 sequentially formed on a substrate 100. A second non-magnetic layer 105, a second pinned layer 106, and a second anti-ferro-magnetic layer 107 are sequentially formed on the free layer 104. The magnetization direction of the first pinned layer 102 is fixed in a first direction by the first anti-ferro-magnetic layer 101, while the magnetization direction of the second pinned layer 106 is fixed in a second direction by the second anti-ferro-magnetic layer 107. The magnetization direction of the free layer 104 may be changed in an arbitrary direction. The first non-magnetic layer 103 is formed of Cu. The first pinned layer 102, the first non-magnetic layer 103, and the free layer 104 have a conventional giant magneto-resistance (GMR) structure.
The second non-magnetic layer 105 is formed of Al, and the free layer 104, the second non-magnetic layer 105, and the second pinned layer 106 have a tunneling magneto-resistance (TMR) structure.
It has been reported that a dual spin filter structure in which a GMR structure and a TMR structure are connected to each other may have a lower critical current density relative to the critical current density of the simple GMR structure and the TMR structure. However, in the dual spin filter structure, the first and second pinned layers 102 and 106 may be arranged in opposite directions, and thus, there is a problem in that the magnetoresistance (MR) disappears. Also, in order to reduce the critical current density of the dual spin structure, the first and second non-magnetic layers 103 and 105 (having different resistances from each other) may be formed or the magnetization direction of the second pinned layer 106 may be arranged in a perpendicular direction to the first pinned layer 102, not in an opposite direction. Regardless, however, compensating for reduced MR is relatively difficult.